1. Field of the Invention
The present invention is related to a transmission system comprising a transmitter with a speech encoder. More particularly speech encoder comprises analysis means for determining analysis coefficients from an input speech signal, the transmitter transmits frames of data representing the speech signal via a transmission medium to a receiver, a fraction of the frames carries more information about said analysis coefficients than the remaining frames, and the receiver comprises a speech decoder for deriving a reconstructed speech signal from the frames of data representing the speech signal.
The present invention is also related to a transmitter, a speech encoder and a speech coding method.
2. Description of the Related Art
A transmission system is known from U.S. Pat. No. 4,379,949.
Such transmission systems are used in applications in which speech signals have to be transmitted over a transmission medium with a limited transmission capacity, or have to be stored on storage media with a limited storage capacity. Examples of such applications are the transmission of speech signals over the Internet, transmission of speech signals from a mobile phone to a base station and vice versa and storage of speech signals on a CD-ROM, in a solid state memory or on a hard disk drive.
In a speech encoder the speech signal is analyzed by analysis means which determines a plurality of analysis coefficients for a block of speech samples, also known as a frame. A group of these analysis coefficients describes the short time spectrum of the speech signal. An other example of an analysis coefficient is a coefficient representing the pitch of a speech signal. The analysis coefficients are transmitted via the transmission medium to the receiver where these analysis coefficients are used as coefficients for a synthesis filter.
Besides the analysis parameters, the speech encoder also determines a number of excitation sequences (e.g. 4) per frame of speech samples. The interval of time covered by such excitation sequence is called a sub-frame. The speech encoder is arranged for finding the excitation signal resulting in the best speech quality when the synthesis filter, using the above mentioned analysis coefficients, is excited with said excitation sequences. A representation of said excitation sequences is transmitted via the transmission channel to the receiver. In the receiver, the excitation sequences are recovered from the received signal and applied to an input of the synthesis filter. At the output of the synthesis filter a synthetic speech signal is available.
The bitrate required to describe a speech signal with a certain quality depends on the speech content. In case the analysis coefficients are substantially constant over a prolonged period of time, the bitrate required to transmit them could be reduced. This possibility is used in the transmission system according to the above mentioned U.S. patent. This patent describes a transmission system with a speech encoder in which the analysis coefficients are not transmitted every frame. They are only transmitted if the difference between at least one of the actual analysis coefficients in a frame and a corresponding analysis coefficient obtained by interpolation of the analysis coefficients from neighboring frames exceeds a predetermined threshold value. This results in a reduction of the bitrate required for transmitting the speech signal. In the known transmission system the bitrate can be set to arbitrary values by increasing or decreasing the threshold value, resulting in a decrease or increase of the bitrate. However the average bitrate still strongly depends on the speech content.